1. Field of the Invention
The present invention relates to a spread spectrum communication synchronization acquisition decoding apparatus which is mainly used at a reception portion of a radio communication apparatus for spread spectrum (SS) communication, and performs synchronization acquisition and synchronization tracking of received signals and spreading codes in a direct sequence (DS) type spread spectrum communication.
2. Description of the Related Art
A direct sequence (DS) type spread spectrum (SS) communication has been more increasingly studied and developed in a cellular mobile communication field of car phones, etc. because of little interference between cellular phones and a large communication capacity.
A mobile radio communication is bidirectionally performed between a base station having a transmitter and a receiver and a mobile unit having a transmitter and a receiver by setting a radio channel between the base station and the mobile unit.
In this case, the transmitter basically performs digital modulation (primary modulation) such as QPSK, QAM, PSK or FSK modulation or the like on data such as audio data, image data, program data or the like which are digitized, generates a spreading code such as a pseudo noise signal (PN code) or the like in a spreading code generator, and then multiplies the digitally-modulated (primary-modulated) data (information signal) by the PN code thus generated to perform a spread spectrum modulation (secondary modulation). Further, the spread-spectrum-modulated (secondary-modulated) data is subjected to a phase modulation with predetermined carrier, and transmitted as a spread spectrum (SS) signal from an antenna.
On the other hand, the receiver basically receives the spread spectrum (SS) signal by an antenna, and converts it to an intermediate frequency signal or a base band signal. Thereafter, the synchronization of the spreading code used in the spreading code generator of the transmitter is acquired in a synchronizing circuit, and the same spreading code as the transmitter is transmitted to an despreading decoder. In the despreading decoder, the spread spectrum (SS) signal and the spreading code from the synchronizing circuit are multiplied to perform an despread demodulation, and finally the data are demodulated in an information demodulator. In the synchronizing circuit of the receiver, it is required to search a point which is coincident in phase with the transmitted spreading code and limit the timing of the phase-coincidence point in a predetermined range in order to acquire and keep the synchronization.
FIG. 3 is a block diagram showing the basic construction of a synchronizing circuit which is used in a receiver for the conventional spread spectrum (SS) communication.
In the synchronizing circuit, a received spread spectrum (SS) signal is converted to a base band signal by a local oscillator 1, a mixer 12 and a low pass filter (LPF) 2 which are prepared at the reception side, then the base band signal is sampled by a sample hold circuit 3 to obtain a sampled signal, and then the sample signal is transmitted to a correlator 4 which comprises a matched filter. The correlator 4 multiplies one period of the spreading code of the received spread spectrum (SS) signal and one period of a pre-prepared spreading code every chip, calculate the sum thereof and transmits the sum result to a synchronization detector 10. Here, the local oscillator 1, the mixer 12 multiplying the received signal and a local oscillated frequency signal and the low pass filter (LPF) 2 acts as a signal converter for converting the received signal to the base band signal.
FIG. 4 is a schematic diagram showing the basic construction of the correlator 4. The correlator 4 successively stores the spread spectrum (SS) signal converted to the base band signal into a shift register 4a every chip, and also it generates a spreading code sequence by a coefficient generator 4b and multiplies the spreading code sequence by a coefficient generator 4b and the spread spectrum (SS) signal stored in the shift register 4a every chip by a multiplier 4d. The multiplication result is transmitted to an adder 4c to calculate the sum of the multiplication result and output the sum result. When the spreading code sequence from the coefficient generator 4b and the spreading code of the received spread spectrum (SS) signal are coincident with each other in timing, the output of the adder 4c becomes maximum (matched pulse). This matched pulse is detected by the synchronization detector 10, and the despread demodulation is performed by using the above synchronization information.
In the case of the synchronizing circuit for the spread spectrum (SS) communication receiver as described above, if there are a lot of noise, an erroneous judgment of the synchronization position would occur more frequently. Therefore, in order to take a countermeasure to the noise, the output of the matched filter is added and summed up by the amount corresponding to several symbols, and then integrated over plural slots. However, the synchronizing circuit has another problem that when the synchronizing timing varies, it cannot immediately follow the variation if the integration time is long.